Some embodiments of the present disclosure relate to devices for sensing vibrations in earth formations. More specifically, embodiments of the present disclosure are directed to detecting seismic signals utilizing electrodynamic sensing devices, such as geophones and seismometers. These electrodynamic sensing devices are configured or designed for the cancellation of spurious frequency responses in the detection of high frequency seismic signals thereby facilitating the generation of seismic waveforms having expanded frequency range. Of course, practitioners of skill in the art will recognize that embodiments of the present disclosure may be applicable to other types of vibration transducers, for example, used either in sensing or transmitting operations.
In the oil and gas industry seismic sensors are deployed at various locations, such as on the earth surface, in the sea, at the seabed, or in a borehole, to aid in the determination of operationally significant subsurface structural and formation property information by measuring seismic signals reflected from changes in these subsurface formations. In this, seismic sensors are commonly used for purposes of obtaining useful data relating to acoustic impedance contrasts in subsurface structures. In certain applications, such as hydraulic fracture monitoring, cross-well tomography, seismic operations (among other applications not expressly listed), seismic sensors are used to acquire high frequency seismic data.
In seismic signal detection, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors. The output from these sensors is then used to determine the structure of the underground formations or to understand source mechanisms of the acoustic events caused in the target rocks. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic signals from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. For example, the sensed seismic signals may be direct signals that are derived from micro-seismicity induced by fracturing or reservoir collapse or alteration, or reflected signals that are derived from an artificial source of energy.
Sensors generally fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense the particle velocity field in the surrounding media arising from a seismic source.
Geophone seismic sensors are widely used to detect seismic waves. When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface, in the sea or at the seabed, or on the wall of a borehole which penetrates the earth (among other examples), moves with the motion of the surrounding media caused by acoustic wave propagation. A geophone may be designed to detect very small seismic signals, for example, in the order of 10−8 m/s. The typical electrical noise floor is less than 1 micro volt rms. To observe and detect small seismic signals, it is desirable that the seismic sensor have a relatively high sensitivity in combination with a low noise level.
As depicted in FIG. 1A, a typical geophone 10 has one or more detecting systems, such as cylindrical moving coils 12 and magnets 15. The geophone 10 may also include a housing 14 and end caps 18. These moving coils 12 may be suspended by springs 20 so as to be disposed around a magnet 15 having pole pieces 16.
Each moving coil 12 is maintained at a neutral, rest position by the springs 20, and is free to oscillate in a magnetic field of the magnet 15 from a centered position thereof. Springs 20 are usually made from sheet metal designed to maintain the coil 12 at a centered, equilibrium position relative to the magnetic field of the magnet 15. For example, in a geophone that is designed for vertical operation, the springs 20 are pre-stressed to centralize the moving coil 12 in the magnetic field against gravitational acceleration. The pair of springs and the moving mass of the coil operates as a spring-mass system.
As depicted in FIG. 1A, the moving coil 12 is suspended in a magnetic flux field and generates electrical signals that are proportional to the velocity of the moving coil 12 relative to the magnetic field. These electrical signals also correspond to the movement of the ground (media) surrounding the geophone. When the coil 12 moves in the magnetic field, a voltage is induced in the coil that can be output as a signal. The output signal is linear to the velocity of the ground movement above the natural frequency defined by the spring-mass system.
FIG. 1B is a schematic depiction of a response mechanism of a geophone seismic sensor in which x0 is the neutral position of the moving coil, x is the position of the coil in motion and ξ is the relative displacement of the coil against the center of the magnetic field. If the axis of the geophone is aligned with the direction of motion, the moving coil mounted on the spring inside the geophone substantially stays in the same position, resulting in the relative motion of the coil with respect to the housing.
At frequencies above the natural frequency of the spring-mass system, the spring is functionally invisible and the coil generally stays in the same position while the housing moves relative to the coil. At very low frequencies, the housing and the coil substantially move together and the geophone does not output any significant signals. At the natural frequency, the spring-mass system creates resonance. The shunt resistor attached to the geophone controls this resonance by allowing current to flow into the coil and inhibiting the movement of the coil. By adjusting the amount of shunt, the spring-mass system is typically critically damped.
The amplitude and phase responses of the output of a geophone with a shunt resistance as shown in FIG. 1B are:
                    Amplitude        =                              a            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                                          S                ⁡                                  (                                      ω                                          ω                      0                                                        )                                            2                                                                                            (                                      1                    -                                                                  ω                        2                                                                    ω                        0                        2                                                                              )                                2                            +                                                (                                      2                    ⁢                                                                                  ⁢                    D                    ⁢                                          ω                                              ω                        0                                                                              )                                2                                                                        Equation        ⁢                                  ⁢        1                                          Phase          =                                    tan                              -                1                                      (                                          2                ⁢                D                ⁢                                  ω                                      ω                    0                                                                              1                -                                                      ω                    2                                                        ω                    0                    2                                                                        )                          ⁢                                  ⁢                  where          ⁢                      :                                              Equation        ⁢                                  ⁢        2                                D        =                              D            0                    +                                    S              0              2                                      2              ⁢                                                          ⁢              m              ⁢                                                          ⁢                                                ω                  0                                ⁡                                  (                                      r                    +                                          R                      s                                                        )                                                                                        Equation        ⁢                                  ⁢        3                                S        =                              S            0                    ⁢                                    R              s                                      r              +                              R                s                                                                        Equation        ⁢                                  ⁢        4                ω[1/s]: Angular frequency f=2πω    ω0 [1/s]: Angular natural frequency f0=2πω0     D0 [−]: Open circuit damping    D [−]: Total damping    S0 [V/(m/s)]: Open circuit sensitivity    r [ohm]: Coil resistance    m [g]: Moving mass    Rs [ohm]: Shunt resistance
FIG. 1C shows exemplary amplitude and phase responses of a geophone based on the parameters shown below by using Equations 1, 2 and 3.
Parameters
    f0 [Hz]=20    D0 [−]=0.36    S0 [V/(m/s)]=79    r [ohm]=1500    m [g]3.14    RS [ohm]=21734    D [−]=0.7
In seismicity monitoring, it is also desirable to minimize or eliminate spurious responses that may be output by a geophone when sensing seismic signals. As schematically illustrated in FIGS. 2A-2C, such undesirable spurious responses may be due to a rocking or cross-axial movement of the moving coil within the geophone. The spurious responses may interfere with the normal sensing of ground movement by the geophone.
For example, the geophone springs are primarily designed to provide freedom of movement of the moving coil in an axial direction, as shown in FIG. 2A. Although the springs restrict or inhibit radial movement of the coil, in some cases, radial movement of the coil will occur. As shown in FIG. 2B, high frequency resonant responses may be caused when the central axis of the moving coil is laterally displaced from the axis defining the axial movement of FIG. 2A at the center of the geophone. In other cases as shown in FIG. 2C, the moving coil may also move in a rotational mode about an axis perpendicular to the axis defining the axial movement of FIG. 2A.
In particular, spurious responses may be present at the high frequencies that are detected by a seismic sensor (see the note in FIG. 2D). The appearance of a spurious response is dependent on the type and configuration of geophone being used. Referring generally to FIG. 2E, this graph shows two examples of spurious responses varying in frequency and amplitude. For a particular design of a geophone, the spurious response may be in a very narrow frequency band.
FIG. 2D shows exemplary spurious response signals in seismic waveforms detected by a geophone seismic sensor. The spurious response frequency may generally be about 20 to 30 times the natural frequency. Geophone manufacturers typically design their geophones so that the spurious responses are extended to above and outside of the seismic frequencies of interest. However, spurious responses may slightly change the amplitude and frequency of the detected waveforms depending on the orientation of the geophone and the angle of the incident waves. For example, if a geophone is tilted, the moving coil is displaced from the center of the housing due to the gravitational acceleration (note again FIG. 2B). As discussed above with reference to FIGS. 2A-2C, since the springs of the seismic sensor are stiff in relation to lateral movement of the moving coil, the frequency is high and damping is small, i.e., a high Q. Spurious responses appear as a ringing response of sinusoidal waves triggered by the first motion. As a result of these types of situations, problems due to spurious responses are known in wideband seismic waveform detection and recording.
In seismic data acquisition, the useful frequency range is generally from the natural frequency of the geophone to the spurious response. As mentioned above, the spurious response is typically located beyond the useful seismic frequency signals for a particular geophone seismic sensor due to the manufacturer's design. For example, typical seismic signals that are detected in land seismic operations involve a 5 to 70 Hz bandwidth, and usually no useful energy is present beyond 100 Hz, because 1) the seismic source does not generate higher frequencies; 2) the frequency attenuation is high at higher frequencies; and 3) anti-aliasing filters are used to mask the spurious responses.
However, in certain applications and seismic operations, such as shallow seismic surveys, operations that use high frequency sources, shallow VSP work, hydraulic fracture mapping or cross-well tomography, the seismic signals of interest include frequencies even beyond 1000 Hz. For example, in particular in hydraulic fracture mapping, in which the seismic events caused by the fracturing of rocks are detected and recorded, the size of the fracture(s) may be estimated by analyzing the high frequency contents of the recorded waveforms.
Furthermore, in applications such as cross-well tomography, seismic signals are generated by a downhole source that is deployed in a well, and the signals are detected by seismic receivers that are deployed in an adjacent well. Since the distance between source(s) and receivers is not far, the attenuation of high frequencies is small and it is possible to detect and record high frequency content in the seismic waveforms.
Therefore, it would be desirable to provide seismic sensor systems that are designed to minimize or eliminate spurious response signals, for example, in the detection of wideband seismic waveforms. Of course, applications of embodiments of the present disclosure are not limited to this exemplary desire.
Another interest is in the signals in low frequencies. If there is an interest to detect a low frequency event lower than the natural frequency of a geophone, the amplitude of the signal is very small and may be submerged or overwhelmed by the electronic noise. For example, a natural earthquake can contain signals below 1 Hz (called sub hertz). To detect such low frequency information, it is desirable to build a low frequency geophone (possibly called a seismometer). If a 1 Hz geophone is built, the spurious frequency may be around 20 Hz. This would fall within the frequency range of interest.
The limitations of conventional seismic sensor designs noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known sensor mechanisms. The above should be sufficient, however, to demonstrate that sensor structures existing in the past will admit to worthwhile improvement.